Batteries that are charging can give off H2 gas. The batteries have Sulfuric Acid inside them and if you overcharge the batteries the H2 gas can be given off.

Natural gas has an additive put into it to make it smell. They put some kind of mercaptan in the natural gas lines to give it a strong odor. Even at low levels in the Parts per Million (PPM) range you can smell this mercaptan additive.

So most likely you are smelling the Hydrogen gas from the lead acid batteries.



Regards,

Moose1am